1. Field of the Invention
This invention relates to a device for transporting poison rod assemblies between fuel assemblies in a nuclear fuel storage facility and, more specifically, to a device for transferring a poison rod assembly which has a plurality of poison rod guide combs which are slidable between an open position, where the combs are in an upper spaced apart location, and a closed position, where the combs overlap and are located near the bottom of the device.
2. Description of the Prior Art
Fuel for a nuclear reactor used to create steam and, ultimately, electricity, generally is in the form of fuel rods containing a fissile material. When fuel rods are being stored, the fuel rods are typically supported in nuclear fuel assemblies arranged as spaced parallel arrays. Fuel assemblies are stored in racks in a protective medium, such as water containing boric acid. In addition to the fuel rods, poison rods are disbursed throughout the fuel assemblies to control the fission process. Poison rods generally include a plurality of elongated rods, each containing a neutron absorbent material, which fit in longitudinal openings, or thimbles, defined in the fuel assemblies. The top end of each poison rod is attached to a web thereby forming a poison rod assembly. A T-shaped bar is affixed to the top of the web, creating an easily accessible handle for lifting the poison rod assembly so that it may be transferred from one fuel assembly to another.
Because the poison rods, which are approximately 12 ft. long, are only connected by the web located at the top of the poison rod assembly, the lower end of the poison rods move freely, and, as such, can be difficult to reinsert within a second fuel assembly without the aid of an alignment means. Prior art transfer devices provide comb assemblies as an alignment means. A comb assembly consists of at least two plates having slots corresponding to the rows of poison rods in the poison rod assembly. The plates are oriented at 90 degrees to each other. As such, when the plates are positioned on top of each other, a grid is formed with an opening for a poison rod at each intersection of the slots. To provide extra stability, comb assemblies typically have two spaced grids. Each comb grid is typically formed of four plates, two plates with channels extending in one direction and two plates with channels extending in a perpendicular direction. Each plate will provide support for half of the poison rods. The plates are divided so that the comb assemblies may be moved aside while the web portion of the poison rod assembly is being lifted from the fuel assembly. Once the web portion of the poison rod assembly is above the comb assemblies, the comb assemblies are brought into place, providing support and guidance for the poison rods.
As shown in Hornak et al., U.S. Pat. No. 5,325,408, comb assemblies may be located on pivots attached to a frame. The comb assemblies are located near the bottom of the transfer device so as to provide better alignment of the poison rods during reinsertion into a fuel cell. The comb assemblies may be pivoted out of the way during the lifting of a poison rod assembly and pivoted into place once the poison rod assembly web is sufficiently above the location of the comb assemblies. An alternative method of displacing the comb assemblies can be seen in Beneck et al., U.S. Pat. No. 5,227,125. In Beneck the comb assemblies are pivotally mounted in a housing and attached to springs which bias the comb assemblies into a closed position. An inner sleeve displaces the comb assemblies while the poison rod assembly web passes through the housing. As the web rises above the housing, the sleeve is lifted and the springs lift the comb assemblies into place. In both Beneck and Hornak, the comb assemblies are spaced above the lower end of the housing or frame. As such, a portion of each poison rod will extend beyond the alignment comb assemblies and may become misaligned from the thimbles provided in the fuel rod assembly.
Finally, as shown in Hornak, the transfer device consists of a tubular top portion and a bottom frame portion. The frame portion is typically four vertical L-shaped members held in spaced relation by a plurality of horizontal bands. Due to the length of the frame portion of the transfer device, the frame may become distorted during lifting operations. Any such distortion interferes with the movement of the mechanical parts, including the comb assemblies, of the transfer device.
Therefore, there is a need for a poison rod transfer device having comb assemblies located near the bottom of the transfer device to provide proper alignment between the poison rod assembly and the fuel cell thimbles.
There is a further need for a poison rod transfer device with a frame that cannot be distorted.